This invention relates to a polishing apparatus for making semiconductor wafers with uniform thickness.
ICs and LSIs are formed on refined high-purity silicon wafers. Such wafers are obtained by growing a silicon crystal along an axis and slicing the monocrystal (or single crystal) thus obtained. After a protecting film of silicon oxide is formed on such a silicon wafer, it is coated with a photosensitive resin called photoresist. After a wiring pattern is formed by photolithography, exposed portions of silicon oxide after the photosensitive resin has been removed are taken off by an etching process. An impurity material is diffused to produce impurity regions in the areas where silicon oxide has been removed and the surface of the silicon surface is exposed. If this is repeated many times, many layers of wiring pattern can be formed on the silicon wafer.
Such semiconductor wafers having many layers of wiring patterns formed on a silicon wafer are coming to be used frequently. Since differences in steps tend to appear among wafers if many layers of wiring patterns are formed continuously on a silicon wafer, it is a common practice to polish the surface of a semiconductor wafer (or its protective film of silicon oxide, to be more precise) every time silicon oxide serving as a protective film is formed.
Polishing apparatus of the type so-called chemical mechanical polishing (CMP) apparatus, as shown in FIG. 3, have been in use for polishing wafers having a rigid sphere 34 sandwiched between a top holder member 33a and a bottom holder member 33b. The top holder member 33a is connected to a rotary shaft 31 which is in turn connected to a motor (not shown) serving as a source of rotary motion. The top holder member 33a has throughholes 43 in which bolts 41 are inserted. The outer diameter of the bolts 41 is slightly less than the inner diameter of the throughholes 43. The bottom ends of the bolts 41 are in the form a screw 41a through which the top holder member 33a and the bottom holder member 33b are fastened together. A wafer holder 35 with a hollow interior 35a is attached to the bottom holder member 33b by means of screws 42 (only one shown). The wafer holder 35 has many suction openings 35b through which the hollow interior 35a is connected to the atmosphere outside.
The interior 35a of the wafer holder 35 is connected through a tube 36 to an externally disposed suction pump (not shown). A semiconductor wafer 37, to be polished, is adsorbed to the wafer holder 35 by activating this suction pump. For polishing a target surface of the wafer 37, the CMP apparatus is transported so as to place the adsorbed wafer 37 on a pad 38 with a rotary shaft (shown at 39 in FIG. 4), and both rotary shafts 31 and 39 of the apparatus and the pad 38, respectively, are rotated.
With the rotation of the rotary shaft 31, the top holder member 33a rotates around the solid sphere 34. The rotary motion of the top holder member 33a causes the bottom holder member 33b connected to the top holder member 33a, as well as the wafer holder 35 connected to the bottom holder member 33b, to start rotating similarly. Because of the space for movement (indicated by double-headed arrows in FIG. 3) between the solid sphere 34 and the bottom holder member 33b, as well as that between the throughholes 43 in the top holder member 33a and the bolts 41, the wafer holder 35 has a certain limited degree of freedom of motion. In other words, prior art CMP apparatus polish the entire target surface of a wafer by moving a wafer holder having this limited degree of freedom of motion.
The degree of freedom of motion for the wafer holder of a prior art CMP apparatus was not sufficiently large because it was limited by the range of motion of the bolts 41 in the throughholes 43 and that of the bottom holder member 33b with respect to the rigid sphere 34. While the top holder member 33a is rotating, the bolts 41 may sometimes contact the wall of the throughholes 43, thereby immobilizing the wafer holder 35. Because of the existence of the rigid sphere 34 in between, the applied pressure is not totally communicated to the wafer holder 35. As a result, the surface of the wafer adsorbed to the wafer holder may fail to contact the pad uniformly. In other words, the wafer will not be polished uniformly. In order to polish the wafer uniformly, it was necessary to increase the pressure to be applied.